Miniature optical elements for fiber-optic beam shaping

ABSTRACT

In part, the invention relates to optical caps having at least one lensed surface configured to redirect and focus light outside of the cap. The cap is placed over an optical fiber. Optical radiation travels through the fiber and interacts with the optical surface or optical surfaces of the cap, resulting in a beam that is either focused at a distance outside of the cap or substantially collimated. The optical elements such as the elongate caps described herein can be used with various data collection modalities such optical coherence tomography. In part, the invention relates to a lens assembly that includes a micro-lens; a beam director in optical communication with the micro-lens; and a substantially transparent film or cover. The substantially transparent film is capable of bi-directionally transmitting light, and generating a controlled amount of backscatter. The film can surround a portion of the beam director.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/983,526, filed Nov. 12, 2007, now U.S. Pat. No. 7,813,609.This application also is a continuation-in-part of InternationalApplication No. PCT/US2008/012701, filed Nov. 12, 2008. The entirecontents of the above-identified applications are incorporated byreference herein.

FIELD OF THE INVENTION

This invention relates generally to optical elements, the design andmanufacture of optical elements, and methods of using the same. Inaddition, the invention also relates to using optical elements tocollect data with respect to a sample of interest.

BACKGROUND OF THE INVENTION

Optical analysis methods such as interferometric methods deliver lightonto a sample of interest, and further require collection of a portionof the light returned from the sample. Due to the size and complexity ofmany light sources and light analysis devices, they are typicallylocated remotely from the sample of interest. This is especiallyapparent when the sample of interest is an internal part of a largerobject, such as biological tissue inside of a living organism. Onemethod of optically analyzing internal parts is to guide light from aremote light source onto the sample using a thin optical fiber that isminimally disruptive to the normal function of the sample due to thediminutive cross-section of the optical fiber. An example of such amethod is the optical analysis of a luminal organ, such as a bloodvessel, using a fiber-optic catheter that is connected on one end to alight source outside of the body while the other end is inserted intothe vessel.

A significant barrier to conducting optical analysis of internalregions, such as lumens, is the design and low-cost manufacture ofminiature optical devices for focusing or collimating light. Many typesof optical analysis, such as imaging and spectroscopy, require that thelight incident on the sample be focused at a particular distance orsubstantially collimated. Since light radiating from the tip of astandard optical fiber will diverge rapidly, a miniature optical systemcan be coupled to the fiber to provide a focusing or collimatingfunction. Additionally, it is often desirable to analyze a samplelocation that is not directly in line with the optical axis of thefiber, such as the analysis of the luminal wall of a thin blood vessel.In these situations, a means for substantially altering the direction ofthe light is used in addition to a means for focusing or collimating thelight radiating from the tip of an optical fiber.

Many methods have been previously described for manufacturing miniatureoptical systems suitable for attachment to an optical fiber that providesome of the functionality described above. These methods generallyprovide a beam focusing means using one of three methods: 1) using agraded-index (GRIN) fiber segment; 2) directly shaping the fiber tipinto a lens; or 3) using a miniature bulk lens. A beam directing meansis generally provided using one of four methods: 1) using total internalreflection (TIR) of light from the angled end face of the fiber using anangled, reflective surface; 3) using a miniature bulk mirror; or 4)using a reflective coating on the fiber tip. These methods, however,have numerous inherent limitations, including excessive manufacturingcost, excessive size, or insufficient freedom to select the focal spotsize and focal distance.

There are many miniature optical systems known in the art that can beused for analysis of internal luminal structures. Each optical systemcan be conceptually divided into a beam focusing means and a beamdirecting means. Light is passed from an external light source to theinternal lumen through one or more optical illumination fibers, whichmay be single mode or multimode in nature. The illumination fiber is incommunication with the miniature optical system, which focuses anddirects the beam into the luminal wall. Light is returned from the lumento an analysis apparatus outside the body using the same fiber, or usingother fibers co-located with the illumination fiber. In one type ofminiature optical system design, the focusing means and directing meansare performed by separate optical elements. In another type of design,the focusing means and directing means are performed by the sameelement.

Several features of existing optical systems are undesirable. Forexample, in some devices all of the optical elements must be of adiameter similar to the optical fiber (the diameter often being similarto 125 μm) in order to minimize the overall system size. This greatlyreduces the options available for selecting the focusing element, beamexpander, and beam director and therefore limits the range of focal spotsizes and working distances achievable by the design. Additionally,these extremely small elements are fragile, difficult to handle, andprone to break during manufacturing and operation. Third, in manyembodiments an air gap must be provided in order to use TIR for beamredirection. This requires a tight seal to be maintained between thefiber and the other element to maintain the air gap. This can beproblematic when the device is immersed in water, blood, or stomachacid, or when the device is rotated or translated at high speed in orderto form an image. Fourth, GRIN focusing elements have refractive indexprofiles that are rotationally symmetric, making it impossible tocorrect for cylindrical aberrations induced on the beam. The overalleffect of these drawbacks is that certain miniature optical systems areexpensive, difficult to manufacture, prone to damage, and do not producea circular output at the focal plane.

In addition to the drawbacks listed above, conventional lensed surfacescan only provide small radii of curvature and are largely limited tospherical geometries. Additionally, the beam cannot expand to a sizesignificantly larger than the single mode fiber diameter (often 125 μm)at any point in the optical system. These limitations result in a lenssystem with a limited working distance and significant sphericalaberrations.

As described above, there are significant limitations to currently knownminiature optical systems used for conducting optical analysis orimaging. Accordingly, a need exists for optical elements that overcomethe limitation of existing optical devices.

SUMMARY OF THE INVENTION

In part, the present invention provides a unitary optical element (orcap) having an internal cavity that slides over the end of an opticalfiber for internal or external analysis of a sample. The cap includesintegrated surface features for altering the beam direction as well asfocusing or collimating the light to a prescribed width at a prescribeddistance away from the cap. The cap is sufficiently small to preventdisruption or damage to sensitive samples, such as internal body tissueor luminal organs. Since the cap is a single monolithic element in anembodiment, it can be fabricated using low-cost methods such asinjection molding. A significant cost advantage and improvement inmanufacturing repeatability compared to previously-described methods isachieved.

One embodiment of the invention provides an optical element such as acap, a cover, or an elongated member with a distal curved end face orsurface. The optical element can be manufactured from a single piece ofmaterial that can be fixed to and receive a section of optical fiber.Specifically, the cap has an open end that receives the fiber, a lengthof solid material that is selected to be substantially opticallytransparent, and a closed end with a curved reflective end surface thatacts both as a lens and a mirror. In one embodiment, the curvedreflective surface is shaped to have the focusing properties of a lensand coated to reflect (or partially reflect) incident light.

Light radiates from the optical fiber, travels through the solidmaterial, and impinges on the curved reflective end surface. Thecurvature of the lensed surface can be designed to focus orsubstantially collimate the incident light. The lensed surface can alsobe tilted relative to the propagation direction of the light radiatingfrom the fiber tip. The tilt angle is selected to reflect the light suchthat it exits the cap through a side face and reaches a focus at adesired distance away from the side face.

The reflective property of the distal surface is obtained by coating theexterior of the curved end surface with a reflective material such asmetal or a dielectric material. Additionally, the curvature of thelensed surface can be different along each of two orthogonal axes.Further, the curvature of one axis can be independently adjusted tocompensate for optical distortions imparted on the light as it exitsthrough the substantially cylindrical side face of the cap. Thesingle-piece construction of the cap makes it amenable to manufacturingby low-cost methods such as injection molding.

In one embodiment, the invention relates to an optical beam directingelement. The optical beam directing element includes an elongate unitarycap comprising a cylindrical outer surface having a longitudinal axiscomprising, a proximal endface defining an annular opening and a distalendface comprising a beam directing surface, the elongate unitary capdefining a solid section and a first cavity section defining a volumeextending to a boundary of the solid section, the volume sized tosurround an optical fiber having a fiber endface and receive the opticalfiber, wherein the beam directing surface is angled and positionedrelative to the fiber endface such that light received from the fiberendface is directed a working distance D from the cylindrical outersurface to form a focal spot having diameter w.

In one embodiment, the elongate unitary cap is formed from a materialselected from the group consisting of acrylic, polycarbonate,polystyrene, polyetherimide, polymethylpentene, and glass. D can rangefrom about 0 μm to about 30 mm. In one embodiment, w ranges from about 3μm to about 100 μm. The beam directing element can further include astationary sheath and an optical fiber fixedly disposed within thevolume, the optical fiber and elongate unitary cap arranged to rotatewithin the stationary sheath. In one embodiment, at least a portion ofthe beam directing surface is coated with a reflecting coating. The beamdirecting element can further include a lensed surface disposed withinthe cylindrical outer surface and formed from the boundary. In oneembodiment, the beam directing surface is substantially flat. Thereflecting coating can include a partially transmissive coating.

In one embodiment, the partially transmissive coating splits the lightfrom the fiber endface into a first beam directed to the workingdistance D from the cylindrical outer surface to form the focal spothaving diameter w and a second beam directed to a working distance D′from the cylindrical outer surface to form a focal spot having diameterw′. Further, a beam incident from the fiber endface can be split basedupon the intensity of the incident beam or the wavelength of theincident beam. In one embodiment, a partially reflective coating isdisposed on a distal section of the cylindrical outer surface at aposition such that a beam directed from the beam forming surface passesthrough and reflects back from the partially reflective coating. Apartially reflective coating can disposed within the volume along aportion of the boundary. In one embodiment, the beam directing surfaceis positioned within the volume or the solid section. A second cavitysection can be defined within the solid section such that the beamdirecting surface is partially shielded by a portion of the cylindricalouter surface that surrounds the second cavity section. Further, thebeam directing surface is shaped to substantially remove cylindricaloptical distortion induced by light propagating from the beam directingsurface through the cylindrical outer surface and the stationary sheath.In one embodiment, the beam directing surface is selected from the groupconsisting of biconic asphere, asphere, biconic Zernike, Fresnel, andnon-uniform rational B-spline.

In one aspect, the invention relates to a method of collecting opticaldata from a test sample in situ. The method includes the steps ofproviding an optical fiber including a core, the optical fiber beingadapted to convey an optical beam at a first diameter; providing anelongate unitary cap comprising a cylindrical outer surface and anannular opening that is fixedly and optically coupled to the opticalfiber by receiving and encircling a length of the optical fiber within acavity defined within the cap; and transmitting the optical beam to abeam directing surface such that a first optical beam is directed aworking distance D from the cylindrical outer surface to form a focalspot having diameter w. In one embodiment, the method further includesthe step of splitting the optical beam such that a second optical beamis directed a working distance D′ from the cylindrical outer surface toform a focal spot having diameter w′. In one embodiment, method furtherincludes the step of collecting optical coherence tomography data usingthe first optical beam. In one embodiment, the method further includesthe step of generating one of a reference signal in response to areflecting element disposed within the unitary cap, the reflectingelement acting as an interferometer arm in an optical coherencetomography imaging system. In one embodiment, the method furtherincludes the step of generating one of a calibration signal in responseto a reflecting element disposed within the unitary cap, the calibrationsignal being used to adjust the reference arm path length to match thesample arm path length in an optical coherence tomography imagingsystem.

Summary of Reference Reflector/Scattering Element Embodiments

In one aspect, the invention relates to fiber optic imaging probe havingan elongated section and a proximal and a distal end, the probecomprising a thin controlled optical scattering material applied to thedistal end.

In another aspect, the invention relates to an optical element. Theoptical element includes a membrane or cover having a first surface anda second surface. The membrane includes a polymer and at least oneback-scattering element for controlled optical back-scattering disposedtherein. Further, the membrane allows transmission of substantiallyundistorted imaging light.

The aspects of the invention described herein can include furtherembodiments. For example, the optical element can further include aplurality of back-scattering elements wherein the at least oneback-scattering element and each of the plurality of back-scatteringelements is a particle having a particle dimension, the plurality ofback-scattering elements disposed within the polymer. In one embodiment,the membrane is shaped to form a curved surface suitable for engulfing,surrounding, enveloping or otherwise covering an optical fiber endfaceor micro-lens.

The particle dimension, in some preferred embodiments, is less thanabout 1.5 μm. Further, the particles can include titanium, zinc,aluminum, and/or other materials suitable for scattering light. Theplurality of scattering elements can have a concentration of about 0.1%doping concentration by volume. The optical element can further includean elongate member, wherein the membrane is shaped to form a sheathwithin which the elongate member is disposed to form a portion of aprobe tip.

In one aspect, the invention relates to an optical element. The opticalelement includes a curved e cover having a first surface and a secondsurface, the cover forming a portion of an imaging probe, the covercomprising a polymer and at least one back-scattering element forcontrolled optical back-scattering disposed therein such that areference point is generated for an imaging system from the opticalback-scattering, the cover allowing transmission of substantiallyundistorted imaging light.

In another aspect, the invention relates to an imaging probe. The probeincludes an elongate section having a first end and a second end; thesecond end forming a probe tip capable of intra-lumen imaging, the probetip comprising a scattering material, the elongate section adapted totransmit light reflected by the scattering material to the first end ofthe elongate section.

In one embodiment, the elongate section is an optical fiber. Theelongate section can be a sheath. Also, the probe can further include anoptical fiber disposed within the sheath. The scattering material caninclude a plurality of light scattering particles dispersed in a matrix.The scattering particles can include titanium and/or other materialsknown to scatter light. Also, the matrix can include polyethyleneterephthalate and/or other polymers such as urethane derivatives.

In one embodiment of an aspect of the invention, the controlled amountof backscatter is in an amount of light at least sufficient to generatea reference point in an imaging system for calibration of at least oneimaging system parameter. The substantially transparent film can alsoinclude a plurality of scattering particles.

In still another aspect, the invention relates to a method ofcalibrating an optical coherence tomography system. The method includesgenerating scan data in response to light reflected from a sample, thereflected light passing through a bi-directional substantiallytransparent optical element; generating reference data in response toscattered light reflected from a scattering element disposed within thebi-directional substantially transparent optical element; andcalibrating the optical coherence tomography system to determine therelative longitudinal position of the scattering element.

In one aspect, the invention relates to a method of fabricating anoptical element. The method includes the steps of selecting a materialsuitable for intra-lumen use in an animal; selecting a dopant suitablefor dispersion in the material, the dopant adapted to scatter light inresponse to an optical source; determining a dopant volume concentrationsuch that a radial scan of a doped material generates a definedbackscatter.

One embodiment of the invention provides an optical cap that can befixed to an end of a section of optical fiber, the cap having an openend that receives the fiber, an internal curved surface in line with theoptical fiber that acts as a lens, a length of solid material, and aclosed end with a flat reflective end surface that acts as a mirror. Insome embodiments, the reflective end surface is coated and in otherembodiments it is uncoated. The curvature of the internal lensed surfaceis chosen to focus or substantially collimate light radiating from theend of the optical fiber. The reflective end surface is made to bereflective by coating the exterior of the end face with metal or adielectric material. In one embodiment, the tilt angle theta between theend face and the axis of the fiber will generally be about 45degrees+/−about 20 degrees.

Another embodiment of the invention provides an optical cap that can befixed to an end of a section of optical fiber, the cap having an openend that receives the fiber, an internal curved surface in line with theoptical fiber that acts as a lens, a length of solid material, and aclosed end with a curved reflective end surface that acts as a secondlens and a mirror. The internal lensed surface is curved along one ortwo orthogonal axes to provide a first focusing means acting on lightradiating from the tip of the fiber. The end surface is also curvedalong one or two orthogonal axes to provide a second focusing meansacting on light transmitted from the first lensed surface and throughthe length of solid material. In one embodiment, the end surface is madereflective by coating with a reflective material. In one embodiment, thereflective material may be a metal or a dielectric material. In oneembodiment, the optical cap is a unitary cap. Further, the optical capcan be made from one or more pieces of material in some embodiments.

Still another embodiment of the invention provides an optical cap thatcan be fixed to an end of a section of optical fiber, the cap having anopen end that receives the fiber, a length of solid material, and aclosed end with a curved partially-reflective surface. Light radiatesfrom the tip of the fiber, travels through the solid material, andimpinges on the partially-reflective surface. A portion of the light isfocused by way of the curvature of the surface, is reflected, and exitsthrough a side face of the cap. Another portion of the light isrefracted and transmitted through the end face of the cap. In this way,optical measurements can simultaneously be made along two differentaxes. The end face is made partially reflective by coating the surfacewith a thin layer of metal, a patterned layer of metal, or by coatingwith a thin dielectric film that is designed to partially transmitlight.

An additional embodiment of the invention provides an optical cap thatcan be fixed to an end of a section of optical fiber, the cap having anopen end that receives the fiber, a length of solid material, a closedend with a curved reflective surface, and a side face with apartially-reflecting or backscattering coating. Light radiates from thetip of the fiber, travels through the solid material, and impinges onthe reflective surface. The light is focused by way of the curvature ofthe surface, is reflected, and impinges on the coated side face of thecap. A portion of this light is transmitted by the coating and reaches afocal spot at a desired distance away from the cap. Another portion ofthe light is directly back-reflected or backscattered by the coating andtravels internally back towards the curved end face. The light reflectsagain off the end face, is re-focused, and is partially coupled backinto the end tip of the optical fiber.

In this way, a controlled amount of reflected or backscattered light canbe generated at a known distance from the focal spot, which isadvantageous for use as a calibration signal or interferometric,reference field in analysis techniques such as optical coherencetomography. The end face is made reflective by coating with metal or adielectric material. The side face is made partially reflective bypartially coating with a material such as gold, aluminum, or othermetals, or by coating with a thin dielectric film that is designed topartially transmit light, or by coating with a layer of smallbackscattering particles. Alternatively, the partially reflectiveproperty may be provided by a thin polymer tube that is impregnated withbackscattering particles, the thin polymer tube being fixed over theoutside of the optical cap. The thin polymer tube may be polyethyleneterephthalate (PET), and the backscattering particles may be titaniumdioxide. The reflective coating can also be selected from suitabledielectric reflective coatings. These dielectric reflective coatings caninclude multiple layers of dielectric material. For example alternativelayers of TiO₂ and SiO₂ can be used in some embodiments to form areflective coating.

Yet another embodiment of the invention provides an optical cap that canbe fixed to an end of a section of optical fiber, the cap having an openend that receives the fiber, an internal surface with apartially-reflecting coating in line with the optical fiber, a length ofsolid material, a closed end with a curved reflective surface, and aside face with a partially-reflecting coating. Light radiates from thetip of the fiber and impinges on the internal partially-reflectingsurface. A portion of the light is reflected or backscattered back intothe fiber while another portion of the light is transmitted and travelsthrough the solid material. In this way, a first amount of reflected orbackscattered light can be generated at a known distance from the focalspot. The transmitted portion of the light then impinges on thereflective surface. The light is focused by way of the curvature of thesurface, is reflected, and impinges on the coated side face of the cap.

With respect to this embodiment, a portion of this light is transmittedby the coating and reaches a focal spot at a desired distance away fromthe cap. Another portion of the light is directly back-reflected orbackscattered by the coating and travels internally back towards thecurved end face. The light reflects again off the end face, isre-focused, and is partially coupled back into the end tip of theoptical fiber. In this way, a second amount of reflected orbackscattered light can be generated at a known distance from the focalspot and at a known distance from the internal partially-reflectivesurface, which is advantageous for use as a calibration signal orinterferometric reference field in analysis techniques such as opticalcoherence tomography. The end face is made reflective by coating withmetal or a dielectric material. The side face and internal face are madepartially reflective or backscattering by partially coating with a metalmaterial, or by coating with a thin dielectric film that is designed topartially transmit light, or by coating with a layer of smallbackscattering particles.

In another embodiment, the invention also provides a method for usingthe various embodiments of the optical cap as a component in afiberoptic imaging catheter, the fiberoptic imaging catheter beinginserted into a luminal structure of a living body and connected to anoptical coherence tomography system in order to obtain high-resolutionimages of the luminal structure.

Still another embodiment provides a means for protecting the lensedsurface of the optical cap by partially or completely locating it withinthe body of the cap. The cap may have any suitable geometry and is notlimited to cylindrical shaped caps. Partial protection of the lensedsurface can be obtained by including an extension of the cylindricalbody slightly proximal of the lensed surface. In one embodiment, partialprotection of the lensed surface can be obtained by locating the lensedsurface entirely within the cavity that receives the optical fiber. Itis understood that any of the embodiments described above can bemodified to include partial or complete protection of the lensedsurface. These embodiments of the invention are no limited to protectionrelated features. For example, recessing the lensed surface can make iteasier to guide the cap distally in some embodiments.

The various embodiments described herein relate to subsystems fortransmitting and receiving various types of electromagnetic radiationthat can be directed through an optical fiber or similar waveguide.Accordingly, although reference may be made to radiation, opticalradiation, light, or other types of electromagnetic radiation, theseterms are not intended to limit the scope of the invention and insteadencompass any type of light or electromagnetic radiation that can besent or received by a lens or optical fiber or similar waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below, and the claims. The drawingsare not necessarily to scale, emphasis instead generally being placedupon illustrating the principles of the invention. In the drawings, likenumerals are used to indicate like parts throughout the various views.The drawings associated with the disclosure are addressed on anindividual basis within the disclosure as they are introduced.

FIG. 1 is a two-dimensional cross-sectional schematic diagram depictingan optical subsystem for directing a beam of light along an opticalfiber and through an elongate member defining a cavity according to anillustrative embodiment of the invention.

FIGS. 2A-B are three dimensional diagrams that show a reflective lensedend surface and an optical element or cap, respectively, according toillustrative embodiments of the invention.

FIG. 3 shows an optical cap surrounding an optical fiber, such that thecap is inside of a transparent sheath according to one embodiment of theinvention.

FIG. 4 shows an exemplary range of focal spot sizes and workingdistances for three different lengths of solid material between anoptical fiber tip and lensed reflective surface of an optical capaccording to an illustrative embodiment of the invention.

FIGS. 5 and 6 show an optical cap that includes a transmissive lensedinternal surface and an angled reflective end surface for directing abeam through a side face of a cap and generating a focal spot at adesired distance from the cap, according to an illustrative embodimentof the invention.

FIG. 7 shows an optical cap that includes a curved, partially-reflectivelensed end surface for generating two focused beams directed through aside face and end face of the cap, according to an illustrativeembodiment of the invention.

FIG. 8 shows an optical cap that includes a curved, reflectively lensedend surface for directing the beam out a side face of the cap through acoating disposed on the outer cylindrical surface of the cap accordingto an illustrative embodiment of the invention.

FIG. 9 shows an optical cap that includes reflective or partiallyreflective surfaces in addition to a curved and reflective lensedsurface according to an illustrative embodiment of the invention.

FIG. 10 shows an optical cap in which the reflective lensed surface isprotected from damage by locating it inside a volume defined within thecap, according to an illustrative embodiment of the invention.

FIG. 11 shows an optical cap in which the reflective lensed surface ispartially shielded from damage by locating it partially within a body ofthe cap, according to an illustrative embodiment of the invention.

FIG. 12 shows an apparatus for conducting optical coherence tomographydata collection according to an illustrative embodiment of theinvention.

FIG. 13 shows a second apparatus for conducting optical coherencetomography data collection according to an illustrative embodiment ofthe invention.

FIG. 14A shows a mold for fabricating an embodiment of the invention.

FIG. 14B shows an embodiment of the invention fabricated using the molddepicted in FIG. 14A.

FIG. 15A shows a mold for fabricating an embodiment of the invention.

FIG. 15B shows an embodiment of the invention fabricated using the molddepicted in FIG. 15A.

FIG. 16 is a schematic diagram of the optical fiber tip, with micro lensand protective cover.

FIG. 17 depicts an image taken with a doped plastic lens cover.

DETAILED DESCRIPTION OF THE INVENTION

The following description refers to the accompanying drawings thatillustrate certain embodiments of the present invention. Otherembodiments are possible and modifications may be made to theembodiments without departing from the spirit and scope of theinvention. Therefore, the following detailed description is not meant tolimit the present invention, rather the scope of the present inventionis defined by the claims.

The use of sections or headings in the application is not meant to limitthe invention; each section and heading can apply to any aspect,embodiment, or feature of the invention.

It should be understood that the order of the steps of the methods ofthe invention is immaterial so long as the invention remains operable.Moreover, two or more steps may be conducted simultaneously or in adifferent order than recited herein unless otherwise specified.

Where a range or list of values is provided, each intervening valuebetween the upper and lower limits of that range or list of values isindividually contemplated and is encompassed within the invention as ifeach value were specifically enumerated herein. In addition, smallerranges between and including the upper and lower limits of a given rangeare contemplated and encompassed within the invention. The listing ofexemplary values or ranges is not a disclaimer of other values or rangesbetween and including the upper and lower limits of a given range.

It should be understood that the terms “a,” “an,” and “the” mean “one ormore,” unless expressly specified otherwise.

The foregoing, and other features and advantages of the invention, aswell as the invention itself, will be more fully understood from thedescription, drawings, and claims.

The development of advanced optical analysis or imaging methods such asconfocal microscopy, single- and multi-photon fluorescence imaging,harmonic imaging, optical spectroscopy, and optical coherence tomography(OCT) have had a tremendous impact on industrial inspection, fundamentalbiology studies, and in vivo imaging of animals and humans. Althoughthese methods are dissimilar in many ways, they share a common designfeature that the incident light used to illuminate the sample ofinterest be focused or collimated. Focused light provides manyadvantages over unfocused light, including improved localization ofincident light for obtaining better spatial resolution, and higheroptical power density for generating increased signal levels.

A focused or collimated beam is generated by directing the output of alight source through a series of optical elements that together form anoptical system. The elements of the optical system are selected toachieve a desired focal spot size, which occurs at a desired distance,referred to as the “working distance,” away from the last element in theoptical system. The working distance is shown at an angle in thefigures. This is the preferred way to define working distance (parallelto the direction of beam propagation). One preferred embodiment will usea beam that exits the side of the cap at a forward angle of ˜10 degrees.Each specific optical analysis application will have its own optimalspot size and working distance. Confocal microscopy, for example,requires small spot sizes close to 1 μm. OCT, on the other hand,requires moderate spot sizes of about 5-about 100 μm.

Although it is possible to obtain a wide range of spot sizes and workingdistances using optical systems comprised of conventional bulk lenses,many applications require flexible and miniaturized optical systems inorder to analyze samples located inside of a larger object. Biomedicineis one example of a field where this requirement is often found. Theoptical analysis of luminal structures such as the esophagus,intestines, urinary tract, airway, lungs, and blood vessels can uselight from an external light source that is transmitted via a flexibleprobe, focused with a miniature optical system, and returned through theflexible probe to a data analysis system outside the body.

Furthermore, it is often desirable to analyze the luminal wall insteadof the contents of the lumen, for example imaging the intima and mediaof a blood vessel wall using OCT instead of imaging the blood containedin the vessel. This results in an additional design objective ofdirecting the beam away from the longitudinal axis of the optical systemor along another preferred direction (or range of directions). Thesetypes of optical probes are often referred to as “side-firing,”“side-directed,” “side-imaging,” or “side-looking.” The size of theselumens can be as small as several millimeters, such as in blood vessels,making the design of the miniature optical system quite challenging. Inaddition, the embodiments described herein are also suitable for usewith various multi-fiber or fiber bundle embodiments. The variousembodiments described below address these needs and others associatedwith probe components and beam formation.

Overview

In general, the invention relates to an optical element having anelongated three dimensional shape such as a cap. The optical elementdefines a cavity or channel. The optical element can be sized to receivean optical fiber portion and operatively direct and focus light. Theoptical element can be fixed to an optical fiber and used to bothredirect and focus light outside of the cap and receive light from asample of interest. The present invention provides methods for using theminiature optical cap and fiber as part of an insertable probe, whichcan in turn be used to conduct optical analysis of a luminal structureinside a living body. Other embodiments of the invention also relate tothe design, manufacture, and use of such devices for delivering focusedor substantially collimated light to a sample, and returning a portionof the light from the sample for processing with imaging or datacollection systems. One exemplary non-limiting example of such a systemis an optical coherence tomography (OCT) system.

Beam Forming Elements

FIG. 1 shows one embodiment of the invention suitable for forming a beamat a predetermined location. Specifically, an optical system 10 suitablefor directing and collecting light or otherwise collecting data withrespect to a sample of interest is shown. In the example shown, anoptical fiber is connected on its proximal end to a light source (notshown). The optical fiber includes coated region 12, and a light-guidingcore with cladding region 14. In one embodiment, the coated region 12includes a polyimide material. As shown in FIG. 1, the coating has beenpartially removed to expose a portion of the core and cladding distal tothe coated region 12. A protective material 16 such as an adhesive alsosurrounds the core and cladding 14 and/or the coated region 12 as shownin FIG. 1. The optical fiber guides optical radiation from a lightsource to the distal fiber segment, where a length of the coating hasbeen removed by mechanical or chemical stripping. The fiber end face(alternatively, fiber tip) can be flat or, to prevent aberrations andunwanted backreflections, can be cleaved to a small angle, typicallybetween about 8° and about 15°. This cleave operation can be performedwith a fiber cleaver. In one embodiment, since such a cleave operationis quick and consistent it offers cost savings and manufacturingadvantages.

In general, in part, the invention relates to a unitary optical element(alternatively, an optical probe element or cap) 18 formed from atransmissive material. In one embodiment, the unitary optical element orcap is elongate in shape. In other embodiments, the optical element orcap is spherical or semi-spherical. For example, in one embodiment, thecap is a sphere or a partially flattened sphere with a fiber-acceptinghole formed in a non-diametrical direction, specifically a ½ radius downfrom the center of the sphere. However, any suitable cap geometry ispossible. The optical element defines a bore or a channel that extendsthrough a portion of optical element 18 before terminating at a wall orregion 19 formed from the transmissive material. As shown, the fibercore and cladding 14 and coated region 12 are disposed within a volumedefined in the cap 18 and enter the cap 18 through the annular opening17 shown on the left side of the figure. Although the invention relatesto various types and forms of such optical elements that define achannel, cavity or bore that partially encircles or surrounds an opticalfiber, the terms “cap,” “cover,” “optical assembly,” “beam former,”“lens assembly,” or other terminology may be used in a non-limitingmanner herein.

Thus, in one embodiment the optical cap (alternatively, an opticalelement) 18 includes a fiber containing section 20 and a beam forming(or solid) section 21. Since a continuous unitary material is typicallyused, a conceptual boundary shown by dotted line or boundary 22delineates the first section 20 from the second section 21. The boundary22 of the optical cap 18 can be imagined as defining a plane positioneddistal to the adhesive or protective material 16 that fills the voidbetween the core 14 and the optical cap 18. As shown, in one embodimenta gap is present between the fiber endface 23 (which may be tilted fromabout 8° to about 15°) and the cavity wall 19. In addition, as shown theadhesive or protective material 16 fills the cavity which includes thecoated region 12 and core with cladding 14. The optical element has acurved surface 25 that is distal to the fiber core with cladding 14. Theclosed distal face/curved endface 25 can include one or more coatings,such as reflecting or partially reflecting coatings. In addition, theoptical element and fiber assembly is typically disposed within a sheath28. In one embodiment, the optical element 18 and the other elementsconnected or fused thereto rotate together relative to the sheath. Inanother embodiment, both the sheath 28 and the optical element 18rotate. In another embodiment, the sheath and optical element 18 arefixed and do not rotate. Also, the region between the sheath and opticalelement can be filled with fluid.

In one preferred embodiment, the optical element is a monolithic orunitary material. Although combinations of materials can be used to makethe optical element, such as mixtures of polymers or glasses, in generalthe composition of the element is designed to be substantially the samethroughout in one embodiment. Coatings or other materials may be appliedto, fused with, or otherwise coupled or connected to the unitary opticalelement.

As shown in the embodiment of FIG. 1, the optical fiber core withcladding 14 and coated region 12 is inserted into a cavity on theproximal side of the optical element 18. Proximal and distal refer tolocation relative to the end of the fiber that is connected to theinstrumentation outside of the body. The cavity (alternatively, fiberreceiving chamber of the optical element) defined by wall 19 can befilled with a protective material or adhesive 16 as shown. The adhesive16 is selected to be substantially optically transparent, and can becured using ultraviolet light, heat, exposure to air, or any othercuring method. To reduce the possibility of bubble formation between thefiber end face 23 and the cavity wall 19, application of the adhesive 16may be performed under a partial vacuum. The material or adhesive 16 maybe chosen to have a refractive index close to that of the fiber 14 andthe optical element 18 in order to reduce backreflections.Alternatively, the material or adhesive 16 may be chosen to have arefractive index different than that of the fiber 14 and the opticalelement 18 in order to produce backreflections with a controlledamplitude. In one embodiment, the adhesive is an acrylic based adhesive.In one embodiment, an ultraviolet light curable adhesive is used.

In one embodiment, the size of the cavity is chosen to be very close tothe size of the fiber to prevent tilt issues. The fiber end face isplaced in contact with the end of the cavity to prevent longitudinalalignment issues. In one embodiment, this material 16 is an adhesivehaving a refractive index similar to that of the optical fiber core andthe material used to form the element 18, such that back reflectionsfrom the fiber tip 23 are further reduced.

When the adhesive is cured (such as by exposure to heat, light, orultraviolet radiation) the fiber becomes fixed to the cap in the volumeor cavity shown. Alternatively, the cap can be formed in place over topof the optical fiber core with cladding 14 and coated portion 12 using aprocess such as injection molding. Molding the cap directly onto thefiber removes the gluing step and can result in reduced manufacturingcosts. Thus, in some embodiments, region 16 comprises the same materialfilling region 18. That is, when no adhesive 16 is used, the regiondefined in FIG. 1 is removed and the cap directly contacts the fiber.

The cap is in the general form of a cylindrical tube with a closeddistal face. The outer diameter of the optical element 18 is typicallyon the order of 2× the diameter of the optical fiber, giving an outerdiameter range of about 160 μm to about 500 μm. In turn, the innerdiameter of the optical element 18 can range from about 80 μm to about250 μm.

In one embodiment, the cap 18 is made from a single piece of material,chosen to be optically transparent in the spectral band used for theparticular imaging or analysis application. In general, the opticalelements or caps described herein are suitable for use with imagingapplications that use wavelengths of electromagnetic radiation thatrange from about 350 nm to about 2000 μm. To facilitate low-cost andhigh-volume manufacturing, the material can be a resin or polymerinstead of a glass. If low aberration levels and high transmission aredesired for a given application, the cap can also be formed out ofglass. Preferred materials include acrylic, polycarbonate, polystyrene,polyetherimide, or polymethylpentene. These materials can be injectionmolded into parts on the size scale of the optical cap using methodsknown in the field of micro-molding. Further, these materials aresuitable for forming a unitary cap. In general, some embodiments of theelongate unitary cap include an optically transmissive material. As usedherein optically transmissive material means a material with lowabsorption and scattering in the spectral band used for the specificapplication, such that a substantial fraction of the light radiatingfrom the optical fiber is transmitted.

In one embodiment, a single piece, molded part provides a significantreduction in manufacturing cost and time, and improvement inpart-to-part uniformity, compared to miniature optical systems alreadyknown in the art as described above. In one embodiment, the length ofthe optical cap ranges from about 0.25 mm to about 5 mm. The gap betweenwall 19 and endface 23 ranges from about 0 μm to about 1000 μm.

Light traveling along the optical fiber exits the fiber end face 23 andcavity wall 19 and propagates a length L into the solid material of thesecond section 21 of the optical element 18. The length L is equal tothe distance from the cavity wall 19 to the center of the closed distalsurface 25. As the light travels, it will diverge as shown by the firstset of dashed lines. Upon reaching the closed distal surface 25, thelight interacts with a coating deposited on the outer surface of thedistal face.

The coating is designed to be highly reflective in the spectral bandused for the particular imaging or analysis application. The coating canbe a metal, a single dielectric layer, or a multi-layer dielectricstack. A non-optically-functional layer may be deposited between thedistal face and the reflective coating in order to improve adhesion. Forexample, such a layer can include chrome, titanium, or a dielectric. Anadditional non-optically-functional layer may be deposited on top of thereflective coating in order to protect the coating from oxidation,peeling, or other damage.

A perpendicular to the center of the distal face 25 is oriented at atilt angle θ relative to the longitudinal axis of the cap, such that thereflected light is directed at an angle 2θ relative to the incidentlight (see FIGS. 2A and 3). The distal face 25 is additionally curved toform a focusing surface, the specific details of which are describedbelow. The light begins to converge or is collimated after interactingwith the distal face 25 and reflective coating. As the light passesthrough the side face of the cap 29 and the sheath 28, it is affected bycylindrical distortion due to the substantiated cylindrical shape of thecap and sheath. This distortion causes the beam to become ovular incross-section instead of circular, and creates a different focal planefor each of the beam's two major axes. These two focal planes areseparated in space along the direction of beam propagation.

Cylindrical distortion is detrimental for many optical analysisapplications, since it leads to anisotropic lateral resolution,decreased peak incident power density, and degraded axial resolution.The curvature of the distal face 25, however, can be different in thetwo orthogonal axes lying in the plane of the distal face, which enablesthe lens to be optimized to pre-compensate for cylindrical distortionbefore it occurs. In this way a circularly symmetric beam can beobtained outside of the cap, and the undesirable effects of cylindricaldistortion can be avoided. Details of the distal face 25 geometry aredescribed fully below.

Once the light exits the cap, it continues to converge until it reachesa focal plane or focal spot at a working distance D away from theclosest edge of the optical element 18. When the fiber is a single-modefiber, the beam is Gaussian and its size at the focal plane is definedby the focal diameter w which is equal to twice the radius of theGaussian profile of the beam. In one embodiment, the length L and thegeometry of the distal face can be selected to give a wide range offocal spot sizes and working distances. In one embodiment, D is measureas the distance from the side of the cap to the focal plane, along thedirection of beam propagation (not necessarily normal to the cap). Thisapproach is consistent with the manner D is illustrated in FIG. 1.

If a long working distance is desired for a particular application, thelength L can be increased to allow the beam to expand to a largerdiameter prior striking the distal face 25. The beam can expand up to amaximum diameter equal to the outer diameter of the cap, which can rangefrom about 160 μm to about 500 μm. Increased beam expansion on thedistal face is equivalent to increasing the aperture of the opticalsystem, which allows the working distance D to be increased for a givenfocal diameter w. If a small focal diameter w is desired for aparticular application, the radius of curvature of the distal face canbe decreased. This effectively increases the focal power of the opticalsystem.

End Face Geometry

FIG. 2A shows a three-dimensional perspective thawing of the lensedsurface 40 (see surface 25 in FIG. 1) with a first focal point F₁ and asecond focal point F₂. FIG. 2B shows a three-dimensional perspectivedrawing of the entire optical element or miniature optical cap 50,having an outer diameter A, and inner diameter D_(i), and overall lengthB. As shown, the cap includes an annular opening 17 sized to receive andfixedly couple to an optical fiber. The focusing or beam forming surface40 from FIG. 2A is implemented as the surface 52 of FIG. 2B in oneembodiment.

In general, in one embodiment, the optical elements are designed to formor direct a substantially circular symmetric beam substantially free ofdistortions outside of the optical element or cap. To facilitate thisdesign feature, the distal face surface is chosen to have differentcurvatures along the arcs traced out by rays Ax and Ay that correspondsto the curves C₁ and C₂, respectively, on the surface 40. Ax and Ayoriginate from different focal points F₁ and F₂, respectively. Multiplesurface geometries 25, 40 are suitable for the optical elements/capsdescribed herein, including biconic asphere, biconic Zernike, Fresnel,or non-uniform rational B-spline. A biconic asphere surface is generallysuitable for applications requiring focal spot sizes of about 3 μm toabout 100 μm and working distances of about 0 μm to about 30 mm, andwhere it is desired to correct for cylindrical distortions caused by theside face of the cap and other materials located between the cap and thefocal plane.

Returning to FIG. 2A, the deviation z of the lensed surface away from aflat plane, such as x-y planes, commonly referred to as the surface sag,is defined by the following equation, for a biconic asphere:

$z = \frac{\frac{x^{2}}{R_{x}} + \frac{y^{2}}{R_{y}}}{1 + \sqrt{1 - \frac{\left( {1 + k_{x}} \right)x^{2}}{R_{x}^{2}} - \frac{\left( {1 + k_{y}} \right)y^{2}}{R_{y}^{2}}}}$When plotted, this equation traces out the shape of the curved surfaceof the lens and that individual z values correspond to varying surfacesag relative to the x-y plane.

In this equation, x, y, and z are local coordinates having an origin Oat the center of the surface. R_(x) and R_(y) are spherical radii ofcurvature along the x and y axes, respectively. In the embodiment ofFIG. 2A, Ax and Ay are examples of Rx and Ry. Additionally, k_(x) andk_(y) are conic constants along the x and y axes, respectively, giving atotal of four free parameters for the surface sag z. The surface 40 isalso rotated about the x axis to an angle θ, in order to direct the beamof the surface at an angle 2θ. The surface can also be offset in theydirection by an amount y_(off) in order to further reduce aberrations inthe optical system.

Optimization of Design Parameters

According to FIG. 3, an optical system 70 including a beam directingsurface 72 for directing the beam out a side face of the cap 75 andgenerating a focal spot at a desired distance from the outer surface ofthe sheath 76 is shown. Thus, in some embodiments, the beam directingsurface both directs and focuses a beam of light or other radiation.Additionally, the curvature of the lensed surface 72 can be adjusted tocompensate for distortions caused by transmission through the sheath 76and the outer cylindrical surface of the cap 75. The following sectiondescribes a process for designing a miniature optical cap of the typeshown in FIG. 3 in order to achieve a desired focal spot size andworking distance that is optimized for a specific optical analysisapplication. The optical analysis application chosen for thisillustrative example is OCT imaging of the coronary blood vessels, whichrequires the optical fiber and miniature optical cap to rotate andtranslate longitudinally. This approach offers numerous advantages.These advantages include:

-   -   Cost savings from reduced manufacturing time and removing the        need for fusion splicing    -   Ability to provide a non-rotationally-symmetric lens shape to        compensate for cylindrical distortion    -   Potentially improved repeatability in focal spot size and        working distance

FIG. 3 shows a miniature optical cap 75 enclosed within a flexible,transparent sheath 76 having an inner diameter M and a wall thickness T.The fiber core with cladding 14, coated region 12, and cap 75 rotate andtranslate within the sheath by means of an external actuator, while thesheath 76 remains stationary in the blood vessel to prevent damage tothe vessel wall.

In this illustrative example, the desired focal spot diameter w isapproximately 30 μm. It is desired that the beam reach a focal plane ata distance D′ away from the side face of the sheath 76 and a distance Daway from the cap, where D′ is about 1.6 mm and D is about 1.857 mm. Thesheath wall thickness T is about 102 μm and the inner diameter M isabout 710 μm. To allow sufficient clearance between the cap and theinner surface 77 of the sheath, the cap outer diameter A is chosen to beabout 400 μm. In one embodiment, the cap material is chosen to beacrylic, since this material is optically clear at one wavelength ofinterest of about 1310 nm.

In this example, to avoid unwanted specular back reflections from theinner surface 77 of the sheath 76, the distal face tilt angle θ ischosen to be about 50° such that the incident light impinging on thedistal face is redirected at an angle of about 100° relative to thelongitudinal axis of the fiber. The angle θ is shown as being formedbetween the longitudinal axis of the fiber and a normal vector to thesurface 72. Light therefore strikes the inner surface of the sheath 77at about an angle 10° off normal incidence, and specular backreflectionsare avoided. In one embodiment, the lumen 78 between the cap and thesheath is filled with radio-opaque contrast fluid having a refractiveindex of approximately 1.449. Further, in one embodiment the lumenbetween the sheath 76 and the blood vessel wall is filled with the samecontrast material or saline. The contrast material may be provided by aproximal flushing mechanism in order to temporarily displace blood fromthe vessel and enable a clear OCT image.

The design parameters remaining to be optimized are the distance L fromthe fiber tip to the lensed surface, the surface sag parameters R_(x),R_(y), k_(x) and k_(y), and they offset y_(off) (if any). An opticalsimulation tool, such as ZEMAX (ZEMAX Development Corporation, Bellevue,Wash.) or an equivalent tool can be used to find the optimal combinationof the remaining design parameters to produce the desired focal spotdiameter of about 20 μM at a distance of about 1.4 mm from the sheath.This software can also be used to ensure that the resulting beamstriking the blood vessel wall is circular and free of aberrations. Toaccomplish this, an iterative optimization algorithm is employed thatsearches for the best combination of free parameters to minimize thevalue of a user-defined error function.

The error function measures several properties of the simulated beam atthe focal plane along the local x′ and y′ axes (See FIG. 3). Themeasured values are compared to the desired values, and a weighted sumof the difference between the measured and desired values is generatedto give the value of the error function. The error function incorporatessimulated values of the beam radius at the 13.5% intensity level(corresponding to the characteristic radius, ω₀, of the beam) along thex′ and y′ axes (Rx and Ry) corresponding to the characteristic beamwaist ω₀, Gaussian fit along the x′ and y′ axes (Gx and Gy), anddistance between the desired focal plane and the actual focal planealong the x′ and y′ axes (Fx and Fy). The error function is constructedto give a zero value when the beam diameters along the x′ and y′ axesare equal, the Gaussian fit is achieved, and the distance between thedesired focal plane and the actual focal plane along the x′ and y′ axesare equal to zero. When the beam diameters along the x′ and y′ axes areequal, the beam is circularly symmetric which is desirable for producingan isotropic focal spot. When a Gaussian fit is achieved along the x′and y′ axes, the system has minimal distortion which is desirable formaximizing image quality and optimizing the amount of optical powerreturned to the system for analysis. When these conditions are met, thebeam is substantially free of aberration and has the desired focal spotsize w at the desired working distance D.

In this illustrative example, the error function E may be chosen toincorporate six parameters, including Rx, Ry, Gx, Gy, Fx, and Fy. Eachparameter is additionally assigned a weight W1 through W6, so as tocontrol the relative importance of each parameter in the error functionE. Each parameter is also assigned a target value Rxt, Ryt, Gxt, Gyt,Fxt, and Fyt. Rx, Ry, Rxt, Ryt, Fx, Fy, Fxt, and Fyt that can bemeasured in units of millimeters. Gx, Gy, Gxt, and Gyt are unitlessparameters that fall within the range of 0 to 1, with 1 representing aperfect Gaussian fit. The error function E is defined as the weightedsum of each parameter minus its corresponding target value, such thatE=W1(Rx−Rxt)+W2(Ry−Ryt)+W3(Gx−Gxt)+W4(Gy−Gyt)+W5(Fx−Fxt)+W6(Fy−Fyt).

In this illustrative example, Rxt and Ryt may be 0.017 mm, correspondingto a full-width-at-half-maximum beam diameter of 0.020 mm. Gxt and Gytmay be 1. Fxt and Fyt may be 0. W1 and W2 may be 50, W3 and W4 may be0.1, and W5 and W6 may be 1. Each choice of optical design values L,R_(x), R_(y), k_(x), k_(y), and y_(off) results in a set of beamparameters Rx, Ry, Gx, Gy, Fx, and Fy that in turn results in aparticular value for the error function E. With the parameter targetsand weights selected, one or more approaches can be used to determinethe combination of optical design values L, R_(x), R_(y), k_(x), k_(y),and y_(off) that results in a minimum error function. This can beachieved by finding either a local minimum in the error function, or aglobal minimum in the error function. Many optical design packages, suchas ZEMAX, contain built-in optimization algorithms that are adequate forperforming this step.

In this illustrative example, the results of the optimization processgive a value for L of about 721 μm, R_(x) of about −772 μm, R_(y) ofabout −1675 μm, k_(x) of about −3797 μm, k_(y) of − about 15,970 μm, andy_(off) of about −23 μm. With these values implemented in an opticalfiber cap embodiment, a focal spot size can be formed of about 29.6 μmin diameter at a distance D′ of about 1600 μm. Such a focal spot size issuitable to perform OCT imaging and data collection in the coronaryblood vessels.

Exemplary Focal Spot Sizes and Working Distances

Embodiments of the optical components such as the caps or beam formingelements described herein enable a wide range of focal spot sizes andworking distances by building miniature optical caps with differentdistal face surface geometries and different distances L (L₁, L₂, andL₃) between the fiber tip and distal face. L₁, L₂, and L₃ are chosen forillustrative purposes and do not limit the scope of the invention. Inone embodiment, L₂ was chosen to be half of L₁, and L₃ is half of L₂. Asan illustrative example, an acrylic cap with an outer diameter of about400 μm is used to depict various data points as shown in FIG. 4.Specifically, FIG. 4 illustrates a subset of the design parametersavailable for an acrylic cap as discussed above, where the distance Lfrom the fiber tip to the distal end face is chosen to be one of about2.10 mm, about 1.05 mm, or about 0.56 mm, respectively. These particularvalues of L are chosen for illustrative purposes only and do not limitthe scope of the invention. At each point on each curve, the end facegeometry was optimized according to the method described above, using anerror function as described above, in order to obtain a focal spot at agiven working distance.

FIG. 4 shows that for this particular type of end cap, focal spot sizesof about 4.3 μm to about 110 μm can be achieved at working distances ofabout 0 mm to about 11 mm. For any given length L, the maximum workingdistance D′ occurs when the focal spot size approaches the size of thebeam incident on the distal end face. Under this condition, the focusingpower of the optical system is weak and the working distance cannot befurther extended.

Internal Lensed Surface

FIG. 5 shows another optical subsystem 80 of the present invention,where beam focusing is provided by an internal lensed surface 83 at theend of the cavity instead of by the distal end face. The cap 85 has adifferent cavity shape because of the additional lensed surface 83. Thesurface 83 can be concave or convex, can have different radii ofcurvature in the x and y axes, can be spherical or aspheric or any othertype of surface generally known in the art of lens design. The surface83 allows for beam collimation or beam focusing and other features. Ingeneral the shape of surface 83 may be of the same form as the shapeshown in FIG. 2A. Surface 83 can be any type of lens surface. Thesurface 83 can be of the same form as the shape shown in FIG. 2A, wherethe tilt angle theta can be as low as zero degrees. This surface 83serves as a boundary between the cavity and beam forming sections of thecap 85 akin to boundary 19 in FIG. 1. Beam direction is still providedby the distal end face 25′, although in this embodiment the end face isangled and flat.

In one embodiment, the end face 25′ is made reflective by coating itwith a reflective material such as metal or a dielectric coating. Inthis embodiment, light radiating from the fiber tip expands into a gapG. The gap may be filled with an optical adhesive, so as to join thefiber 14 to the cap 85, or it may alternatively be filled with air toallow more rapid beam expansion. The length of the gap G is set by thelength S of the fiber where the protective coating has been removed, andby the length of the cavity S+G.

In one embodiment, a taper 87 on the proximal side of the cavity acts asa stop against the coated portion of the fiber, such that the insertionlength of the fiber can be precisely controlled. Alternatively, acylindrical stop may be used in place of a taper, although taperedfeatures are generally preferred for micro-molding fabricationprocesses. In micro-molding processes, sharp edges are difficult tofabricate. The gap length G and the surface sag of the internal surfacecan be optimized in a manner analogous to the one described above usingan error function. If the cap 85 is to be placed inside of a sheath (notshown), it is understood that the gap length G and the surface sag forsurface 83 can be further optimized to correct for distortions caused bytransmission through the sheath.

This cap embodiment 85 provides several benefits in addition to thosedescribed above for the cap design shown in FIG. 3. First, the lensedsurface 83 is located within a cavity, protecting it from accidentaldamage during handling or operation. Second, the area of the internallensed surface 83 is smaller than that of the distal end face 25′, whichsimplifies the tool design used to fabricate the cap. Third, because thedistal end face 25′ is flat instead of curved, achieving a uniformcoating layer thickness is simplified. Coating adhesion may also beimproved because of the flatness of the end face.

Dual Lensed Surfaces

FIG. 6 shows another optical subsystem 90 with a cap embodiment 95,where beam focusing is provided by a combination of one internal lensedsurface 96 at the end of the cavity and a reflective lensed surface 97formed on the distal end face. Beam direction is still provided bymaking the end face 97 reflective by coating with a reflective materialsuch as metal or a dielectric coating. In this embodiment, lightradiating from the fiber tip 23′ expands into a gap G. The lightrefracts upon interaction with the internal lensed surface 96 andpropagates through a length L′ of solid material. The light thenimpinges on the distal end face 97, where it is further focused andredirected out a side face of the cap. The gap length G, the solidmaterial length L′, and the surface sags of the internal surface 96 anddistal end face 97 can be optimized in a manner analogous to the onedescribed above using an error function. If the cap 95 is to be placedinside of a sheath, it is understood that the gap length G and thesurface sag of both surface 96 and surface 97 can be further optimizedto correct for distortions caused by transmission through the sheath.

This cap embodiment 95 provides several benefits in addition to thosedescribed above for the cap designs shown in FIG. 3 and FIG. 5. First,the use of two lensed surfaces 96, 97 provides more free designparameters and allows a wider range of focal spot sizes w and workingdistances D to be obtained. Second, the use of two lensed surfacesresults in fewer geometric aberrations than a comparable design using asingle lensed surface, improving the optical quality of the resultingbeam. Third, the internal lensed surface 96 and gap G can be configuredsuch that the light transmitted into the length L′ of solid material issubstantially collimated. In this way, the exact value of L′ becomesless critical to the overall optical performance of the system, therebyimproving the design's tolerance to fabrication errors. Under certainmanufacturing conditions, the embodiments show in FIG. 3, 8 or 11 arepreferred embodiments.

Partially Reflecting End Face for Dual Beam Scanning

FIG. 7 shows another system embodiment 100 of the present invention thatuses a cap 105, where light radiating from the tip 23 of the fiber issplit into two focused beams B₁, B₂ by a partially-reflecting coating onthe distal end face 107 of the miniature optical cap 105. In thisembodiment, light radiating from the fiber tip 23 expands into a lengthL of solid material. The light impinges on the distal end face 107,where it interacts with a partially reflecting coating that transmits aportion of the light through the end face and reflects another portionof the light through the side face of the cap. The reflected portion ofthe light B₁ reaches a focal plane at a first working distance D with afocal spot size w. The transmitted portion of the light B₂ reaches asecond focal plane at a second working distance D″ with a second focalspot size w″. If the cap is to be placed inside of a sheath, it isunderstood that the solid length L and the distal end face surface sagcan be further optimized to correct for distortions caused bytransmission through the sheath.

The partially-reflecting coating referenced above can be formed inseveral ways. First, a highly reflective material such as metal can beapplied in a pattern on the distal end face such that the metal coversless than 100% of the end face area exposed by the beam. The pattern caninclude a checkerboard, annulus, concentric rings, or any other pattern.Second, a dielectric coating can be applied over a continuous portion ofthe end face area. The properties of the dielectric material can beselected to partially reflect a fixed fraction of the incident opticalpower. Alternatively, the dielectric coating can be selected tosubstantially reflect one wavelength band and to substantially transmita second wavelength band. This type of coating is commonly referred toas a “dichroic” or “dichroic mirror” coating.

The embodiment of FIG. 7 provides several benefits in addition to thosedescribed above for the cap designs shown in FIG. 3, FIG. 5, and FIG. 6.First, generation of two beams B₁, B₂ along different axes allowssimultaneous analysis of two different sample locations. Thisfacilitates examining luminal structures inside the body. Oneillustrative example is OCT imaging of a blood vessel containing anocclusive lesion. Using this embodiment, forward-looking annular imagescan be obtained at the same time as side-looking radial images byrotating the catheter about the axis of the fiber. In this way, OCTimages can be obtained from in front of the catheter as it is advancedinto the lesion in order to analyze the lesion makeup.

More generally, forward imaging is useful for guiding the placement ofan imaging catheter to avoid perforating a luminal wall. If a dichroiccoating is employed, an additional benefit is the ability to conductoptical analysis of a sample in front of the cap using one group ofwavelengths, and optical analysis of a sample beside the cap using asecond group of wavelengths. Thus, using such an approach it is possibleto conduct multimodal imaging of a luminal structure. OCT imaging can beconducted using about 1310 nm light directed through the side of thecap, while confocal fluorescence imaging can be conducted using about800 nm light directed through the front of the cap.

Fixed Reflection Surfaces

In several optical analysis and data collection applications, includingOCT imaging, it is desirable to include one or more surfaces thatgenerate reflections of known intensity at known positions relative tothe focal plane. This facilitates calibration and interferometercalculation in some embodiments. A fixed reflection can be used in OCTapplications to generate a calibration signal for adjusting thereference arm length to match the sample arm length (See U.S. Pat. App.Pub. No. 2009/0122320, Petersen et al.) The disclosures of which areincorporated by reference in their entirety.). A fixed reflection canalso be used to generate a reference field that interferes with thelight returned from the sample in OCT applications. As a result, thisforms a common-path interferometer within the imaging catheter andavoids the need for a separate reference arm.

In part, the present invention enables the generation of fixedreflections including a calibration signal only, a reference field only,or both a calibration signal and a reference field. FIG. 8 shows asystem 110 embodiment of the invention where a coating 111 is applied toa region of the side face of the miniature optical cap 113. Typically, apartially-reflective or backscattering coating 111 is applied to aportion of the side face of the optical cap to generate a controlledreflection at a known distance from the focal spot. As shown, thecoating 111 overlaps the region of the side face where the beam exitsthe cap 113. The coating 111 is chosen to be partially transmissive,which can be achieved by using a patterned metal coating, a thin-filmdielectric stack, or small backscattering particles. By using thecoating 111, a fixed portion of light will be reflected by the coatedportion of the side face. This reflected light impinges again on thecurved distal face 115 and is coupled back into the optical fiber 14.

The amount of reflected light that is desired to be coupled back intothe fiber 14 depends on whether the fixed reflection will be used togenerate a calibration signal or a reference field. If an OCTcalibration signal is desired, the intensity of the light coupled backinto the fiber 14 from the fixed reflector 115 should be similar to theintensity of light returned from the sample in order to preventsaturation of the detection system.

If an OCT reference field is desired, the intensity of the light coupledback into the fiber 14 from the fixed reflector 115 should be severalorders of magnitude higher than the intensity of light returned from thesample. This provides sufficient heterodyne gain to the sample light andthereby obtains sufficient detection sensitivities for imaging inscattering tissue. However, since the coating 111 is not located at thefocal plane of the optical system, and since it is placed on thecylindrically curved side face of the cap, the back reflected light willnot be perfectly coupled into the fiber. Therefore the reflectivity orbackscattering fraction of the coating is typically selected such thatit is sufficiently high to compensate for these fiber coupling losses,which can be calculated with optical design tools commonly used in thefield.

FIG. 9 shows another system embodiment 120 where two fixed reflectionsare provided by two coatings. Specifically, in one embodiment,partially-reflective or backscattering coatings are applied to a portionof the side face and an internal face of the optical cap 121 forgenerating two controlled reflections at known distances from the focalspot. One coating 123 is located on a portion of the side face of thecap, and the other coating 124 is located on a portion of the internalsurface of the cavity that receives the optical fiber 14. By generatingtwo fixed reflections, the miniature optical cap 14 can provide onecalibration signal and one reference field. Alternatively, twocalibration signals can be provided or two reference signals can beprovided.

Distal Tip Designs for Optical Surface Protection

For some analysis applications, it is desirable to protect the opticalsurfaces of the miniature end cap from damage that may occur duringcatheter assembly or during operational use of the device. FIG. 10illustrates a system 130 that protects the optical surface 131 bydisposing it within the optical element or cap 133. Beam focusing isprovided by an internal lensed surface at the end of the cavity thatreceives the fiber. Beam direction is provided by the same internalsurface by tilting the surface relative to the longitudinal axis of thefiber. The internal surface is made reflective by coating with areflective material such as metal or a dielectric coating.

FIG. 11 shows a system 140 with an optical element 141 that providespartial protection of the optical surface 142, which may be sufficientto prevent damage in many applications. In this embodiment, the opticalsurface 142 is located within a recess foamed by extending thecylindrical wall 144 of the cap distally beyond the end face. In thisembodiment, the fiber 14 resides in a first cavity formed in the opticalelement or cap 141 and the light directing surface 142 is formed in asecond cavity formed at the distal end of the cap 141.

Optical Coherence Tomography Imaging

The various embodiments of miniature optical caps described here arewell-suited for conducting OCT imaging of internal luminal structures. Aflexible OCT imaging catheter can be constructed by enclosing theoptical cap and fiber within a transparent sheath that covers the lengthof the catheter. The fiber and cap can then be rotated about thelongitudinal axis of the fiber to conduct side-directed spiral imaging.Rotational motion can be coupled from a motor outside of the body by useof a torque cable. These various combinations of elements can operate asa data collection probe as shown in the system embodiments of theapplicable figures. Forward-directed annular images may also be obtainedif the cap is configured to generate a forward-looking beam in additionto the side-looking beam, as shown in FIG. 11.

FIG. 12 shows a data collection system 150 for conducting OCT imagingwith a flexible catheter that includes a miniature optical cap 155 atits distal tip. The optical cap 155 is fixed to a flexible optical fiber153 to form an insertable imaging catheter, which directs light onto asample 157 and returns sample arm light to the OCT interferometer. Alight source is in optical communication with an OCT interferometer,which can be a Michelson interferometer or any variant thereof that isknown in the field. The light source can be a broadband superluminescentdiode, a tunable laser with a narrow instantaneous linewidth and broadtuning range, a supercontinuum source, or any source of low-coherenceoptical radiation. The OCT interferometer is in optical communicationwith a reference aim, which generates a reference field that interfereswith sample light returned from the sample arm.

The sample arm comprises an optical coupler and a flexible imagingcatheter. The optical coupler connects to the proximal end of thecatheter, directing a portion of the radiation from the light sourceinto the catheter. The optical coupler also provides rotational andtranslational motion, which is translated to the distal tip of thecatheter and the miniature optical cap. Light is guided down the fiber,focused and redirected by the miniature optical cap 155, and impinges onthe sample. As shown, the fiber and cap combination can rotate.Backscattered and back reflected light from the sample is collected bythe miniature optical cap and transmitted back down the fiber, throughthe optical coupler, and into the OCT interferometer. The sample andreference arm light interferes, and is then detected, processed, anddisplayed by a data acquisition and display system.

FIG. 13 shows another system 170 for conducting OCT imaging with aflexible catheter that includes a miniature optical cap 173 at itsdistal tip, the optical cap having at least one fixed reflecting surfaceas shown in FIG. 8 or FIG. 9. The optical cap is fixed to a flexibleoptical fiber 175 to faun an insertable imaging catheter, which directslight onto a sample and returns sample arm light to the OCTinterferometer. Additionally, the optical cap 173 generates a fixedreference reflection at a known position relative to the focal plane.The reference reflection acts as an interferometric reference field andinterferes with the sample light to form optical coherence tomographyimage lines. This configuration is known as a “common path”interferometer in the field of OCT imaging. Common path interferometersembodiments offer the advantage of matching optical aberrations such aschromatic and polarization-induced dispersions in the sample andreference arms as they are common mode (the sample and reference fieldsare generated after traveling through a substantially the same physicalpath). In turn, matching these types of aberrations improves imageresolution and contrast. However, certain types of common-mode noise canno longer be cancelled. Overall, the benefits of a common-pathinterferometer often outweigh the disadvantages, once a practical methodfor building the common-path design has been established.

In this case, the fixed reflecting surface is configured to produce areference field that interferes with the sample light returned from thesample. The optical coupler and flexible catheter therefore comprise anintegrated reference arm and sample arm. This arrangement has manybenefits compared to the apparatus shown in FIG. 12. First, since thereis no separate reference arm, the system cost and complexity arereduced. In a traditional OCT interferometer, a reference arm having thesame length as the sample arm is required. In this embodiment, thereference field is generated very close to the sample and therefore thereference arm path length is inherently matched to the sample arm pathlength.

Manufacturing Process and Mold Embodiments

Any of the embodiments of the present invention can be manufactured froma single piece of material in one step or multiple steps, followed byapplication of coatings in subsequent steps. Alternatively, multiplepieces of material can be joined together in single or multiple steps.To achieve a small manufacturing cost and rapid manufacturing time, themanufacturing process can be of any type of molding, including injectionmolding, compression molding, or a specialized type of injection moldingknown as micro-molding. FIG. 14A shows a mold 200 containing fourcomponents used to produce an embodiment of the present invention with amolding process. A large two-part clamshell can be used to form theelongate cylindrical shape of the cap. A first core pin, with a with agradual change in diameter from approximately the outer diameter of thecoated fiber region to the outer diameter of the core and claddingregion, can be used to form the cavity that receives the optical fiber.A second core pin, with a diameter approximately equal to the diameterof the closed end face of the cap, can be used to form the opticalsurface at the end of the cap. Essentially, the core pins are disposedin the mold and the material which will form the final molded part flowsand solidifies around the pins and the FIG. 14B shows a molded part 205that can be obtained using the mold tool 200 shown in FIG. 14A.

Optical-quality surface finishes can be attained by diamond-turning theends of the core pins. This process reduces aberrations from surfaceroughness and thereby improves image quality. Furthermore, the use ofcore pins allows the optical surfaces to be formed from a single moldedpiece, rather than machining half of the optical surface on each of thetwo clamshell mold pieces that fowl the cylindrical body of the cap.Alternatively, the first core pin may be replaced in the micro-moldingprocess by the optical fiber itself. This arrangement, known in thefield as “molding in place” or “over-molding”, positions the opticalfiber in half-cavities formed in the two clamshell components of themold.

FIG. 15A shows a mold 210 containing three components used to produce anembodiment of the present invention with an over-molding process. In anover-molding process, the optical fiber takes the place of the core pinthat would have formed the cavity that receives the optical fiber.During the molding process, the melted polymer flows directly over thefiber and hardens in place, forming the molded part directly on thefiber. Since this process incorporates the fiber into the molded part,it precludes the need for gluing or separately joining the fiber intothe cavity of the molded part during subsequent assembly steps. FIG. 15Bshows a molded part 215 that can be obtained using the mold tool shownin FIG. 15A, where the optical fiber is joined to the elongate capdirectly during the over-molding process.

Integrated Reference Reflector and Scattering Particle Embodiments

FIG. 16 depicts an embodiment of the image wire tip of the probe. Theoptical fiber 270 terminates in a micro-lens assembly 326 which focusesthe light at a distance from the micro-lens assembly 326. Light emittedfrom the micro-lens assembly 326 is reflected by a beam deflector 330 soto as to pass at substantially right angles to the optical axis of thefiber 270. The entire fiber assembly is covered by a protectivetransparent sheath 334 which has been doped with a small amount ofscattering material to provide a reference reflection corresponding tothe sample arm path length. This reflection is most useful in anon-common-path interferometers (the more typical type), as the sampleand reference optical paths are physically distinct, yet must bepath-length matched to create the required interference signal.

Several materials exist as a suitable dopant. In particular titaniumdioxide (TiO₂) is advantageous. TiO₂ is used in many paint formulationsdue to its excellent light scattering properties. Further it is inertand can be made in bulk. The particle size can be made much smaller thanthe optical wavelengths of interest (nominally 1.3 μm), making thescattering ‘Rayleigh’ in nature. Thus the outgoing and returning lightwavefronts are not appreciably disturbed, thereby minimizing anypotential image degradation at sufficiently low concentrations ofdopant.

In addition, because OCT imaging has tremendous sensitivity and largedynamic range (typically 100 dB of sensitivity and >60 dB of dynamicrange can be achieved in practical instruments) care must be used tocalculate then achieve the optimal doping level of TiO₂ in the material.

Basic scattering theory can be used to arrive at a doping concentrationin the material. In a typical OCT image in the coronary arteries, theminimum noise in the instrument is about −100 dB. That is, about 1ten-billionth of the optical output power applied to the object ofinterest and a typical image has approximately 40 dB of useful dynamicrange. The image processing electronics and software are optimized forthis range, so the probe reflector element should be optimized to benear the maximum detectable peak of the image intensity, which is about−60 dB (−100+40). This means that the probe reflector should be thebrightest object in the image.

As described herein the probe reflector element can include, but is notlimited to, a membrane, a film, a cap, a cover, or other material. Insome embodiments, the reflector element is flexible or inflexible. Thereflector element can be shaped in various geometries, such thatportions of the reflector are curved, planar, or substantially planar.

Basic scattering theory for particles and classic radar cross-sectiontheory estimates that the fraction of light reflected from a single TiO₂particle is given by the expression:

$L_{R} = {\frac{\sigma_{b}}{V_{i}}l_{c}{\Delta\Omega}}$where L_(R) is the return light fraction, σ_(b) is the scatteringcross-section (calculated from standard MIE theory), V_(i) is the volumeof the particle, l_(c) is the interaction length (from Radar theory),this case the coherence length of the OCT light, and ΔΩ is acceptanceangle (solid angle) of the micro-lens. Thus, for a particle size ofroughly 45 nm with a scattering cross section of approximately 4.26×10⁻⁷μm², and light having a coherence length of about 15 μm irradiating theparticle through a micro-lens having a solid angle of ˜0.004, thereflected light fraction, L_(R), is about 0.006, or −32 dB.

Therefore the total light returned from the probe reference reflectorelement material should be equal to the single particle light fractiontimes the volume fraction (doping concentration). Because this should beequal to about −60 dB (from above), a reduction of −30 dB (or 0.001) isrequired. Therefore, the volume fraction should be about 0.001, or about0.1% doping concentration by volume. This should result in a strong, butnot overpowering reference reflection by the TiO₂ particles, as shown inFIG. 17.

Having thus described certain embodiments of the present invention,various alterations, modifications, and improvements will be apparent tothose skilled in the art. Such variations, modifications andimprovements are intended to be within the spirit and scope of theinvention. Accordingly, the foregoing description is by way of exampleonly and is not intended to be limiting.

What is claimed is:
 1. An optical beam directing element comprising: anelongate unitary cap comprising a cylindrical outer surface having alongitudinal axis comprising, a proximal endface defining an annularopening and a distal endface comprising a beam directing surface, theelongate unitary cap defining a solid section and a first cavity sectiondefining a volume extending to a boundary of the solid section, thevolume sized to surround an optical fiber having a fiber endface andreceive the optical fiber, wherein the beam directing surface is angledand positioned relative to the fiber endface such that divergent lightreceived from the fiber endface is directed a working distance D fromthe cylindrical outer surface to form a focal spot having diameter w. 2.The optical beam directing element of claim 1 wherein the elongateunitary cap is formed from a material selected from the group consistingof acrylic, polycarbonate, polystyrene, polyetherimide,polymethylpentene, and glass.
 3. The optical beam directing element ofclaim 1 wherein D ranges from about 0 μm to about 30 mm.
 4. The opticalbeam directing element of claim 1 wherein w ranges from about 3 μm toabout 100 μm.
 5. The optical beam directing element of claim 1 furthercomprising a stationary sheath and an optical fiber fixedly disposedwithin the volume, the optical fiber and elongate unitary cap arrangedto rotate within the stationary sheath.
 6. The optical beam directingelement of claim 1 wherein at least a portion of the beam directingsurface is coated with a reflecting coating.
 7. The optical beamdirecting element of claim 6 further comprising a lensed surfacedisposed within the cylindrical outer surface and formed from theboundary.
 8. The optical beam directing element of claim 7 wherein thebeam directing surface is substantially flat.
 9. The optical beamdirecting element of claim 6 wherein the reflecting coating is apartially transmissive coating.
 10. The optical beam directing elementof claim 9 wherein the partially transmissive coating splits the lightfrom the fiber endface into a first beam directed to the workingdistance D from the cylindrical outer surface to form the focal spothaving diameter w and a second beam directed to a working distance D′from the cylindrical outer surface to form a focal spot having diameterw′.
 11. The optical beam directing element of claim 10 wherein the beamincident from the fiber endface is split based upon the intensity of theincident beam or the wavelength of the incident beam.
 12. The opticalbeam directing element of claim 1 wherein a partially reflective coatingis disposed on a distal section of the cylindrical outer surface at aposition such that a beam directed from the beam forming surface passesthrough and reflects back from the partially reflective coating.
 13. Theoptical beam directing element of claim 1 wherein a partially reflectivecoating is disposed within the volume along a portion of the boundary.14. The optical beam directing element of claim 1 wherein the beamdirecting surface is positioned within the volume or the solid section.15. The optical beam directing element of claim 1 wherein a secondcavity section is defined within the solid section such that the beamdirecting surface is partially shielded by a portion of the cylindricalouter surface that surrounds the second cavity section.
 16. The opticalbeam directing element of claim 5 wherein the beam directing surface isshaped to substantially remove cylindrical optical distortion induced bylight propagating from the beam directing surface through thecylindrical outer surface and the stationary sheath.
 17. The opticalbeam directing element of claim 5 wherein the beam directing surface isselected from the group consisting of biconic asphere, asphere, biconicZernike, Fresnel, and non-uniform rational B-spline.
 18. A method ofcollecting optical data from a test sample in situ, the methodcomprising the steps of: providing an optical fiber including a core anda fiber endface, the optical fiber being adapted to convey an opticalbeam at a first diameter; providing an elongate unitary cap comprising acylindrical outer surface and an annular opening that is fixedly andoptically coupled to the optical fiber by receiving and encircling alength of the optical fiber and the fiber endface within a cavitydefined within the cap; and transmitting divergent light from the fiberendface to a beam directing surface such that a first optical beam isdirected a working distance D from the cylindrical outer surface to forma focal spot having diameter w.
 19. The method of claim 18 furthercomprising the step of splitting the optical beam such that a secondoptical beam is directed a working distance D′ from the cylindricalouter surface to form a focal spot having diameter w′.
 20. The method ofclaim 18 further comprising the step of collecting optical coherencetomography data using the first optical beam.
 21. The method of claim 18further comprising the step of generating one of a reference signal inresponse to a reflecting element disposed within the unitary cap, thereflecting element acting as an interferometer arm in an opticalcoherence tomography imaging system.
 22. The method of claim 18 furthercomprising the step of generating one of a calibration signal inresponse to a reflecting element disposed within the unitary cap, thecalibration signal being used to adjust the reference arm path length tomatch the sample arm path length in an optical coherence tomographyimaging system.